The respiratory system (or ventilatory system) is the biological system of an organism that introduces respiratory gases to the interior and performs gas exchange. In humans and other mammals, the anatomical features of the respiratory system include airways, lungs, and the respiratory muscles. Molecules of oxygen and carbon dioxide are passively exchanged, by diffusion, between the gaseous external environment and the blood. This exchange process occurs in the alveolar region of the lungs.[1] Other animals, such as insects, have respiratory systems with very simple anatomical features, and in amphibians even the skin plays a vital role in gas exchange.

Most of the respiratory tract exists merely as a piping system for air to travel in the lungs, and alveoli are the only part of the lung that exchanges oxygen and carbon dioxide with the blood.

Moving down the respiratory tract starting at the trachea, the tubes get smaller and divide into more and more tubes. There are estimated to be about 20 to 23 divisions, ending up at an alveolus.

Even though the cross-sectional area of each bronchus or bronchiole is smaller, because there are so many, the total surface area is larger. This means there is less resistance at the terminal bronchioles. (Most resistance is around the 3-4 division from the trachea due to turbulence.)

Most of the epithelium (from the nose to the bronchi) is covered in pseudostratified columnar ciliatedepithelial cells, commonly called respiratory epithelium. The cilia beat in one direction, moving mucus towards the throat where it is swallowed. Moving down the bronchioles, the cells get more cuboidal in shape but are still ciliated.

Cartilage is present until the small bronchi. In the trachea they are C-shaped rings, whereas in the bronchi they are interspersed plates.

Glands are abundant in the upper respiratory tract, but there are fewer lower down and they are absent starting at the bronchioles. The same goes for goblet cells, although there are scattered ones in the first bronchioles.

Smooth muscle starts in the trachea, where it joins the C-shaped rings of cartilage. It continues down the bronchi and bronchioles, which it completely encircles.

Instead of hard cartilage, the bronchi and bronchioles are composed of elastic tissue.

Horses

Horses are obligate nasal breathers which means that they are different from many other mammals because they do not have the option of breathing through their mouths and must take in oxygen through their noses.

Amphibians

Both the lungs and the skin serve as respiratory organs in amphibians. The skin of these animals is highly vascularized and moist, with moisture maintained via secretion of mucus from specialized cells. While the lungs are of primary importance to breathing control, the skin's unique properties aid rapid gas exchange when amphibians are submerged in oxygen-rich water.[5]

Fish

In most fish respiration takes place through gills. (See also aquatic respiration.) Lungfish, however, do possess one or two lungs. The labyrinth fish have developed a special organ that allows them to take advantage of the oxygen of the air.

Anatomy in invertebrates

Insects

Air enters the respiratory systems of most insects through a series of external openings called spiracles. These external openings, which act as muscular valves in some insects, lead to the internal respiratory system, a densely networked array of tubes called tracheae. The scientific tracheal system within an individual is composed of interconnecting transverse and longitudinal tracheae which maintain equivalent pressure throughout the system. These tracheae branch repeatedly, eventually forming tracheoles, which are blind-ended, water-filled compartments only one micrometer in diameter.[6] It is at this level of the tracheoles that oxygen is delivered to the cells for respiration. The trachea are water-filled due to the permeable membrane of the surrounding tissues. During exercise, the water level retracts due to the increase in concentration of lactic acid in the muscle cells. This lowers the water potential and the water is drawn back into the cells via osmosis and air is brought closer to the muscle cells. The diffusion pathway is then reduced and gases can be transferred more easily.

Insects were once believed to exchange gases with the environment continuously by the simple diffusion of gases into the tracheal system. More recently, however, large variation in insect ventilatory patterns have been documented and insect respiration appears to be highly variable. Some small insects do demonstrate continuous respiration and may lack muscular control of the spiracles. Others, however, utilize muscular contraction of the abdomen along with coordinated spiracle contraction and relaxation to generate cyclical gas exchange patterns and to reduce water loss into the atmosphere. The most extreme form of these patterns is termed discontinuous gas exchange cycles (DGC).[7]

Molluscs

Molluscs generally possess gills that allow exchange of oxygen from an aqueous environment into the circulatory system. These animals also possess a heart that pumps blood which contains hemocyaninine as its oxygen-capturing molecule. Hence, this respiratory system is similar to that of vertebrate fish. The respiratory system of gastropods can include either gills or a lung.

The breathing rate increases with the concentration of carbon dioxide in the blood, which is detected by peripheral chemoreceptors in the aorta and carotid artery and central chemoreceptors in the medulla. Exercise also increases respiratory rate, due to the action of proprioceptors, the increase in body temperature, the release of epinephrine, and motor impulses originating from the brain.[9] In addition, it can increase due to increased inflation in the lungs, which is detected by stretch receptors.

Under normal conditions, the diaphragm is the primary driver of inhalation. When the diaphragm contracts, the ribcage expands and the contents of the abdomen are moved downward. This results in a larger thoracic volume and negative pressure (with respect to atmospheric pressure) inside the thorax. As the pressure in the chest falls, air moves into the conducting zone. Here, the air is filtered, warmed, and humidified as it flows to the lungs.

Exhalation

The lungs have a natural elasticity: as they recoil from the stretch of inhalation, air flows back out until the pressures in the chest and the atmosphere reach equilibrium.[10]

During forced exhalation, as when blowing out a candle, expiratory muscles including the abdominal muscles and internal intercostal muscles, generate abdominal and thoracic pressure, which forces air out of the lungs.

Upon inhalation, gas exchange occurs at the alveoli, the tiny sacs which are the basic functional component of the lungs. The alveolar walls are extremely thin (approx. 0.2 micrometres). These walls are composed of a single layer of epithelial cells (type I and type II epithelial cells) close to the pulmonary capillaries which are composed of a single layer of endothelial cells. The close proximity of these two cell types allows permeability to gases and, hence, gas exchange.
This whole mechanism of gas exchange is carried by the simple phenomenon of pressure difference. When the air pressure is high inside the lungs, the air from lungs flow out. When the air pressure is low inside, then air flows into the lungs.

Immune functions

Airway epithelial cells can secrete a variety of molecules that aid in the defense of lungs. Secretory immunoglobulins (IgA), collectins (including Surfactant A and D), defensins and other peptides and proteases, reactive oxygen species, and reactive nitrogen species are all generated by airway epithelial cells. These secretions can act directly as antimicrobials to help keep the airway free of infection. Airway epithelial cells also secrete a variety of chemokines and cytokines that recruit the traditional immune cells and others to site of infections.

Metabolic and endocrine functions of the lungs

In addition to their functions in gas exchange, the lungs have a number of metabolic functions. They manufacture surfactant for local use, as noted above. They also contain a fibrinolytic system that lyses clots in the pulmonary vessels. They release a variety of substances that enter the systemic arterial blood and they remove other substances from the systemic venous blood that reach them via the pulmonary artery. Prostaglandins are removed from the circulation, but they are also synthesized in the lungs and released into the blood when lung tissue is stretched.
The lungs also activate one hormone; the physiologically inactive decapeptide angiotensin I is converted to the pressor, aldosterone-stimulating octapeptide angiotensin II in the pulmonary circulation. The reaction occurs in other tissues as well, but it is particularly prominent in the lungs. Large amounts of the angiotensin-converting enzyme responsible for this activation are located on the surface of the endothelial cells of the pulmonary capillaries. The converting enzyme also inactivates bradykinin. Circulation time through the pulmonary capillaries is less than one second, yet 70% of the angiotensin I reaching the lungs is converted to angiotensin II in a single trip through the capillaries. Four other peptidases have been identified on the surface of the pulmonary endothelial cells.

Vocalization

The movement of gas through the larynx, pharynx and mouth allows humans to speak, or phonate. Vocalization, or singing, in birds occurs via the syrinx, an organ located at the base of the trachea. The vibration of air flowing across the larynx (vocal cords), in humans, and the syrinx, in birds, results in sound. Because of this, gas movement is extremely vital for communication purposes.

Temperature control

Panting in dogs,cats and some other animals provides a means of controlling body temperature. This physiological response is used as a cooling mechanism.

Coughing and sneezing

Irritation of nerves within the nasal passages or airways, can induce coughing and sneezing. These responses cause air to be expelled forcefully from the trachea or nose, respectively. In this manner, irritants caught in the mucus which lines the respiratory tract are expelled or moved to the mouth where they can be swallowed. During coughing, contraction of the smooth muscle narrows the trachea by pulling the ends of the cartilage plates together and by pushing soft tissue out into the lumen. This increases the expired airflow rate to dislodge and remove any irritant particle or mucus.

Development

Humans and mammals

Further information: Development of human lung

The respiratory system lies dormant in the human fetus during pregnancy. At birth, the respiratory system becomes fully functional upon exposure to air, although some lung development and growth continues throughout childhood.[12]Pre-term birth can lead to infants with under-developed lungs. These lungs show incomplete development of the alveolar type II cells, cells that produce surfactant. The lungs of pre-term infants may not function well because the lack of surfactant leads to increased surface tension within the alveoli. Thus, many alveoli collapse such that no gas exchange can occur within some or most regions of an infant's lungs, a condition termed respiratory distress syndrome. Basic scientific experiments, carried out using cells from chicken lungs, support the potential for using steroids as a means of furthering development of type II alveolar cells.[13] In fact, once a pre-mature birth is threatened, every effort is made to delay the birth, and a series of steroid shots is frequently administered to the mother during this delay in an effort to promote lung growth.[14]

Coughing is of major importance, as it is the body's main method to remove dust, mucus, saliva, and other debris from the lungs. Inability to cough can lead to infection. Deep breathing exercises may help keep finer structures of the lungs clear from particulate matter, etc.

The respiratory tract is constantly exposed to microbes due to the extensive surface area, which is why the respiratory system includes many mechanisms to defend itself and prevent pathogens from entering the body.